Rebarbative comes to us from the French word rébarbatif ("grim" or "forbidding"), and its etymology can be traced further to the Latin barba ("beard"). I suppose that unshaven faces might be consided "prickly" or "repellent," though that does not stop me from growing the blasted facial appendages.

One might argue that I subconsciously grow facial hair as a deliberate attempt to repel unwanted conversation from passersby, but I prefer to think it is merely to accentuate the Chuck Norris that lurks within me.